In a broadband wireless access system, data transmission between a base station and a MS is made via a “service flow”. The service flow includes a service flow identifier (SF ID) for identifying the service flow between the base station and the MS, a connection identifier (CID) for identifying a connection for delivering service flow traffic, and quality of service (QoS) parameters for service quality and security information for ensuring the security of the service flow.
In general, the service flow establishes one-to-one connection between the base station and the MS, but the MBS is a point-to-multipoint service for transmitting data from one source to a plurality of receivers. Thus, the base station transmits the same data to several MSs via one service flow. That is, when the MBS flow between the base station and the MS is generated, the base station applies the same CID to several MSs for requesting the reception of the MBS data such that the several MSs can simultaneously receive the same MBS data. A coverage which can provide a service by applying one CID to the several MSs is called an MBS zone.
The MBS zone includes several base stations and the several base stations in one MBS zone can provide the same service via one service CID in several cells. Due to such characteristics, the MBS generally transmits common data from one source to a plurality of receivers via a plurality of base stations. In this case, although a supportable user throughput varies according to the capabilities of user equipments and channel statuses between the user equipments and the base station even in a multicast group which receives the same service, a service suitable for a user equipment for supporting a lower throughput should be provided in order to allow all users to receive the MBS data. Therefore, a user equipment having a channel status or capability which can receive a superior service cannot receive the superior service due to other MSs which have inferior statuses in the same group.